This research is concerned with indirect procedures for measuring perceived distance that are based on the interrelation between perceived distance and some other perceptual variable. One method, the head motion procedure, uses the interrelation between perceived distance and an illusory motion of the stimulus concomitant with a motion of the head. Another, the size-distance invariance hypothesis, uses the interrelations between perceived distance and perceived size. Other perceptual interrelations also will be studied. The validity of these indirect procedures will be tested by comparing the measures of apparent distance obtained from the different procedures. The role of the vertibulo-ocular reflex (VOR) in illusory concomitant motion is investigated. The validity of the size-distance invariance hypothesis is examined for the case in which apparent and retinal size changes are in opposite directions and the head motion procedure is applied to the effect of known size on perceived distance and known distance on perceived size. The research is relevant to understanding the factors involved in responses to distance for simulated as well as naturally occurring environments.